The interaction of immunoglobulins with the complement system has been studied. It has been observed that rabbit anti-DNP antibodies fix guinea pig and human complement to varying degrees, in the absence of antigen, depending upon the method of preparation. A technique for preparing non-fixing monomeric immunoglobulin has been developed, and in addition, monomeric anti-DNP has been prepared which fixes complement several hundred fold better than the "nonfixing" sample in the absence of antigen. Apparently the CH2 domains of rabbit IgG can be perturbed, exposing the complement binding site and causing the molecule to trigger the complement cascade, independent of antigen. Complexes of divalent affinity-labelled antibodies have been prepared and fractionated on the basis of size. The binding of these complexes to mouse spleen cells has been investigated. Monomeric IgG does not bind to more than about 5% of spleen cellls, with the assay used. However small complexes containing 2-3 Ig molecules do bind to about 50% of the cells, even though these same complexes are not anticomplementary. Only very large complexes (larger than 6 IgG) bind to a larger fraction of the cells. Covalently linked dimers have been isolated, and will be used to further characterize lymphocyte immunoglobulin receptors.